1. Technical Field
The invention is related to signal processing employing adaptive filters such as equalizers, crosstalk cancellers, echo cancellers, and the like.
2. Background Art
Signal processing employing adaptive filters involves a signal processor having a number of selectable states in which a received signal is processed differently depending upon the state of the signal processor. A suitable feedback algorithm is employed to select the state of the signal processor so as to optimize some measured parameter of the processed signal or to minimize an error. If, for example, the low pass filtering effects of a local area network (LAN) cable are to be removed, then the signal processor may be an adaptive equalizer whose selectable states correspond to different coefficients in a digital FIR filter, or different capacitance values to which an analog equalizer may be set. If the effects of near end crosstalk are to be minimized, then the signal processor is a near end cross talk canceller, for example. The feedback algorithm may be a least mean square algorithm. Alternatively, it may be a genetic algorithm in which each state of the processor is enabled in turn while the signal processor outputs for each state are compared with one another to determine which state produced the best results.
The invention is embodied in an adaptive filtering system for processing a received signal, including a signal processor having plural states to generate a processed signal from the received signal in accordance with a selected one of the states. The processed signal can be either analog or digital. In either case, the processed signal is a many-leveled signal. A slicer produces from the many-leveled processed signal a few-leveled pulse signal (in the simplest case, two levels, a xe2x80x9c0xe2x80x9d or a xe2x80x9c1xe2x80x9d) as an output signal of said adaptive filtering system. An eye-diagram calculator produces from the pulse signal a metric signal corresponding to the separation between leading and trailing edges of a succession of n pulses in the pulse signal superimposed upon one another within a repetitive sampling window. The graphical representation of these superimposed pulses is known as an xe2x80x9ceye-diagramxe2x80x9d and the separation is qualitatively referred to as the xe2x80x9ceye-opening.xe2x80x9d An adaptive controller responsive to the metric signal finds the one state of the signal processor that optimizes the metric signal, and places the signal processor into that state.
In a preferred embodiment, the eye-diagram calculator works as follows: The slicer output, or pulse signal, is used to sample n equally spaced 50% duty cycle clock signals that are frequency locked or nearly-locked to the pulse signal. Each pulse signal edge therefore produces a vector of n samples, each sample will be a 0 or a 1. A plurality of these n element vectors is summed to produce a vector of n sums. The eye-diagram calculator then computes a metric based upon the contrast between the maximum sum of n/2 contiguous sums and the sum of the remaining n/2 contiguous sums in the n-element vector. The contrast may be defined as a difference or a ratio between the two cumulative sums.